1. Field of the Invention
Often continuous processes utilize multiple zones having gas-comprising atmospheres in a substantially enclosed space with the atmosphere of each zone having immediate access to the atmosphere of an adjacent zone. Such processes, for example processes conducted in a continuous furnace, frequently do not generate consistent high quality product because of inability to control fluid flows between furnace zones and at furnace entrances and exits. Inability to maintain fluid flow control within the furnace causes variation in the atmosphere composition or other intensive properties within a given zone over a time period; for example, as furnace fluid flows change, the composition of individual atmospheres within furnace zones can be affected and the segregation between differing zone atmospheres can be disturbed. The present invention provides a method of controlling fluid flows within a process comprised of multiple zones having atmospheres which are interconnected, such as the multiple zones in a continuous furnace, so the intensive properties of particular atmospheres within particular zones and the segregation of differing atmospheres can be maintained. A more consistent quality product is obtained using the method of the present invention. The atmosphere may be defined as comprising a particular fluid composition, e.g., a gas, vapor or mixture thereof located in a particular zone, and having a pressure and a temperature.
2. Background Art
Furnaces are used in continuous processes dealing with a myriad of products ranging from metal and polymer processing to foodstuff processing to electronics manufacture. The method of the present invention can be used wherever it is desired to control the fluid flow in an individual zone of a multiple zone process or the overall fluid flow of a multiple zone process, such as a process carried out in a continuous furnace. The term "continuous furnace" is an art-recognized term having known meaning to an engineer skilled in the art. The method of the present invention can also be used for control in other fluid flow systems, such as continuous food freezers, ducts, and vents, for example. However, since it is not necessary for an understanding of this invention to discuss all such possibilities herein, the principles involved will be discussed in terms of a continuous furnace having multiple zones.
Nowack U.S. Pat. No. 4,298,341, dated Nov. 3, 1981, describes an industrial oven having air recirculating means for minimizing heat loss. A nozzle for feeding hot air is directed downwardly into the oven in the vicinity of the oven access opening to minimize the escape of hot air through the access opening. The invention is directed to a single zone furnace or oven, however, and is not concerned with the control of fluid flow bias (as defined at pages 8 and 9 herein) in a multiple zone furnace. The invention also does not address the problem of infiltration of gas having a different composition from an external location into the furnace.
Petzi U.S. Pat. No. 4,365,954, dated Dec. 28, 1982 describes a continuous electric tunnel furnace for firing ceramic articles. The atmosphere in the furnace comprises an oxygennitrogen mixture. It is desired to decrease the oxygen content progressively from the furnace entrance toward the furnace exit, so that no oxygen is present in the cooling portion of the furnace near the exit. This is accomplished using a combination of nitrogen gas feed into the furnace exit area in an orientation against the direction of article movement toward the exit, and at least one gate (extending from the furnace tunnel roof into the inner space of the tunnel) to provide a braking effect on the velocity of atmosphere movement within the furnace.
Francis, Jr. et al. U.S. Pat. No. 4,448,616, dated May 15, 1984 discloses a process for reducing backmixing or backflow of ambient gases surrounding a heat treating furnace into the furnace entrance and exit openings. Gas jets are positioned at about the top and across the width of at least one of the furnace openings. The problem of controlling fluid flow bias within zones of a multiple zone furnace is not addressed.
Bowes U.S. Pat. No. 4,543,060, dated Sept. 24, 1985 describes a method of preventing air from leaking into the furnace as workpieces enter or exit the furnace. A sensor is placed at at least one end of the furnace and as the workpiece passes the sensor, the sensor sends a signal to the furnace gas supply source which increases the overall gas flow rate to the furnace. Increasing the overall gas flow rate causes more furnace gas to exit the furnace, thus preventing the entrance of ambient air into the furnace. Again, there is no suggestion of any means for controlling the fluid flow bias within the furnace.
Tsai U.S. Pat. No. 4,506,726, dated Mar. 26, 1985 discloses a method of redistributing gas flow within a regenerator using air jets. A regenerator is a form of heat exchanger used in combination with flat glass manufacturing furnaces. The regenerator comprises a gas pervious bed of refractory material, such as a stacked arrangement of bricks. Gas flows through the regenerator are made more uniform using air jet means to counteract longitudinal flow tendencies in the gas distributing space joining a flue to a bed of packing. This invention is concerned with permanently altering the fluid flow rate through a porous/pervious bed leading to a vent rather than with actively controlling fluid flow bias in a relatively open multiple zone enclosed space.
Tsai U.S. Pat. No. 4,496,316, dated Jan. 29, 1985 discloses a method and apparatus for selective control of combustion gas flow in a furnace firing port. A small quantity of pressurized gas is injected generally along the flow path of combustion air in the plenum to alter the amount of combustion air flowing into the firing port. The invention does not concern fluid flow bias in a multiple zone furnace.
Conybear et al. U.S. Pat. No. 4,191,598, dated Mar. 4, 1980 describes a method and apparatus for recirculation of atmosphere in a vacuum furnace. The furnace atmosphere is continually analyzed and replenished as needed. There is no disclosure related to control of fluid flow bias within a multiple zone enclosed space.
Heat treating processes such as carbon steel brazing, stainless steel brazing, annealing, normalizing, glass-to-metal sealing, copper-thick film firing, and decarburizing may all be performed in a continuous double open ended belt furnace. Products processed in furnaces of this type are sensitive to the composition, temperature, and pressure of the volatile fluid or gaseous atmosphere in which they are processed. For example, in brazing, the presence of a gaseous atmosphere comprising oxygen in the furnace typically causes metallic oxides to form on the metal surface to be brazed, and this may cause failure of the braze. Thus, the process atmosphere of a furnace during brazing must be kept below specified levels of both water and oxygen, for example.
A typical continuous furnace comprises one or more zones of specified fluid composition, and frequently comprises more than one temperature zone. The parts to be processed are placed on a continuous moving belt and enter and exit the furnace through an entrance vestibule and exit opening, respectively. The furnace may have several different fluid injection or vent points, and furnace atmosphere fluids may exit from both ends of the furnace and from any intermediate vents. The way in which furnace fluids are partitioned between flowing toward the front or toward the rear of the furnace is an important process variable in continuous furnaces since it directly affects the stability of the process atmosphere in a given furnace zone.
Items which directly affect the partitioning of furnace fluid flows include the following: Location and quantity and direction of fluid flow into the furnace; furnace entrance and exit configuration; part movement through the furnace; furnace placement within a building; furnace slope; winds (room or external); active or passive vents; furnace cycling; atmosphere composition; atmosphere burnoff; furnace loading; furnace temperature; and external ambient air temperature and humidity, for example.
Often upon start-up of a new furnace, the furnace fluid flow partitioning is determined and set. However, as changes in the items such as those listed above occur, the furnace flow partitioning drifts or changes from its original condition. Thus, the conditions under which the product is being processed in the furnace are constantly changing as the atmospheres of the furnace zones are changing. Control of the furnace fluid flow partitioning, meaning the active manipulation of fluid flow parameters within the furnace to obtain or maintain the desired atmosphere conditions within one or more of the zones of the furnace, is needed to ensure guality control of the parts being processed in the furnace.